Apple cultivar effects on codling moth (Lepidoptera: Tortricidae) egg mortality following fumigation with methyl bromide

Abstract

Fumigation experiments with methyl bromide (MeBr) were carried out in New Zealand over several seasons. In 1988-1989 'Braeburn', 'Fuji', 'Granny Smith', 'Red Delicious', 'Royal Gala', 'Gala' and 'Splendour' apples were infested with freshly laid eggs of codling moth, Cydia pomonella (L), and were fumigated at a range of methyl bromide doses. The concentration-time ctsum, which estimates the average methyl bromide concentration over the 120 min of fumigation, was used as the measure of exposure. The complementary log-log of mortality induced by treatment varied linearly with the ctsum, with the same equation for all apple cultivars. This relationship allowed accurate prediction of a 99% lethal value (LC99) for the ctsum that was again, the same for all cultivars. Subsequent experiments in 1998-1989 with 'Red Delicious', 'Fuji' and 'Pacific Rose' apples' were conducted in the absence of codling moth eggs with the aim of examining the regression of ctsum on dose. There were different lines for different cultivars, and different intercepts for different years. The ctsum estimates, at an injected dose of 24 g m-3 could not, however, be distinguished between cultivars. The estimate of ctsum for 'Pacific Rose' at 24 g m-3 cannot be distinguished from the ctsum estimates for cultivars currently approved for export to Japan. The approved treatment for New Zealand apples exported to Japan includes an injected methyl bromide dose of 24 g m-3. For acceptance of this treatment for a new variety, it is sufficient to demonstrate that the ctsum is maintained at levels which lie within or above the range found for existing approved varieties.